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The Forefront of Space Science

Hottest Gas in the Universe Discovered by SUZAKU
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Fig. 2 shows the data actually retrieved by two types of sensors, X-ray CCD camera and hard X-ray detector, onboard SUZAKU. This graph indicates the X-ray energy spectrum (in other words, X-ray radiation amount per energy level) emitted from the cluster. The highest X-ray energy in this graph indicates gas temperature. The average temperature of the clusterís gas is around 100 million deg. C, but the graph shows that there is a component of 300 million deg. C extending to the high-energy level as shown by the red line.

Combination with X-ray image from Chandra satellite reveals that the 300 million deg. C hot gas discovered by our observation is limited to the area of 450,000 light years in diameter within the cluster (area circled by dotted line in Fig. 1, right) and is shinning like a spot light. Therefore the enormous heat energy is confined in this limited area. Even more amazingly, the temperature of the gas discovered (300 million deg. C) is several times higher than that of gases in previously known galaxy clusters. It is even more surprising because the extremely hot gas is hidden in a galaxy cluster that was believed to be gentle (i.e., stable in terms of dynamics). The discovery also means that we have identified the hottest material ever known in the universe.

Figure 2
Figure 2. Observed data of RXJ1347 galaxy cluster by SUZAKU
Horizontal axis shows energy of X-ray while vertical axis shows intensity per X-ray energy level

What is origin of 300 million deg. C gas?

The next question is ďhow was the hottest gas in the universe produced?ĀEHeating gas to 300 million deg. C in the universe is not simple.

Generally, it is thought that gas is heated up when it falls into the gravitational potential of dark matter. The temperature can become higher if the gravitational potential is deeper. The depth of gravitational potential is determined by the amount of dark matter in the galaxy cluster. In the case of RXJ1347, the amount of dark matter is approx. 1015 of the Sunís mass. Even if such huge gravitational potential exists, the temperature increase explainable by this theory is 100 million deg. C at most. The temperature of the hot gas discovered is several times higher. By the way, it is well known that a starís interior is hot due to nuclear fusion. For example, the temperature of the center the Sun, our most familiar star, is around 15 million deg. C. Compared to this, it is easy to see that 300 million deg. C is an incredible temperature.

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